Recently, an increased demand for radio communication has caused significant developments in radio communication systems.
FIG. 1 illustrates an entire radio communication network.
In FIG. 1, a plurality of mobile stations 10A and 10B and a plurality of base stations 20A, 20B, 20C, . . . are illustrated. The base stations 20A, 20B, 20C, . . . are intended for respective areas and each communicate wirelessly with the mobile station having moved into the area for which the base station is intended. The base stations 20A, 20B, 20C, . . . are connected to a network 40 via radio network control devices 30A, 30B, . . . which mediate communication between each of the base stations and the network 40. The mobile stations 10A and 10B placed in separate areas may also communicate with each other via the network 40. Here, the radio communication between each of the base stations 20A, 20B, 20C, . . . and each of the mobile stations 10A, 10B, . . . is based on an OFDM scheme. The description below relates to a distortion-compensated and amplified transmission apparatus configured as a transmission apparatus for each of the base stations 20A, 20B, 20C, . . .
FIG. 2 illustrates a conventional OFDM distortion-compensated and amplified transmission apparatus.
A distortion-compensated and amplified transmission apparatus 100A illustrated in FIG. 2 includes an OFDM modulation section 110, a distortion compensation circuit 120, a D/A converter 131, an orthogonal modulator 132, a power amplifier 133, a directional coupler 134, an antenna 135, an oscillator 136, an orthogonal demodulator 137, and an A/D converter 138.
The OFDM modulation section 110 subjects transmission data from a high-order apparatus to OFDM modulation. The OFDM modulation section 110 includes an S/P conversion section 111, a subcarrier modulation circuit 112, an IFFT section 113, a P/S conversion section 114, a G/I addition section 115, and a waveform shaping section 116. The S/P conversion section 111 subjects the transmission data from the high-order apparatus to serial-parallel conversion. The subcarrier modulation circuit 112 complexly modulates an output of the S/P conversion section 111 to generate a plurality of subcarrier signals. The IFFT section 113 subjects the subcarrier signals to fast reverse Fourier conversion so that a frequency axis is converted into a time axis. The P/S conversion section 114 subjects an output of the IFFT section 113 to parallel-serial conversion to output a signal based on the time axis and obtained by synthesizing signals of phase planes mapped to the subcarriers. The G/I addition section 115 inserts guard intervals (GIs) into the signal in order to mitigate inter-symbol interference in a multipath fading environment. The waveform shaping section 116 filters an output of the G/I addition section 115.
The distortion compensation circuit 120 performs distortion compensation based on a digital pre-distortion (DPD) scheme using an OFDM signal (reference signal) provided by the OFDM modulation section and a feedback signal for a transmission signal provided by the A/D converter 138. The distortion compensation circuit 120 includes an address generation section 121, a delay circuit 122, a subtractor 123, an LUT (Look Up Table) 124, and a complex multiplier 125. The address generation section 121 generates an address required to read a distortion compensation coefficient corresponding to a power value of the reference signal. The delay circuit 122 adjusts timing for the reference signal so that the reference signal and the feedback signal are compared with each other at the same timing. The subtractor 123 calculates an error component corresponding to a difference between the reference signal and feedback signal delayed by the delay circuit 122. The LUT 124 stores the distortion compensation coefficient adaptively updated so as to reduce the error component calculated by the subtractor 123. The complex multiplier 125 multiplies the reference signal by the appropriate distortion compensation coefficient read from the LUT 124 according to the address generated by the address generation section 121.
The D/A converter 131 converts a digital signal into an analog signal. The orthogonal modulator 132 receives a local oscillator signal from the oscillator 136 to orthogonally modulate and convert an I/Q signal output by the D/A converter 131 into a radio frequency.
The transmission signal converted into the radio frequency by the orthogonal modulator 132 is input to the power amplifier 133, which then amplifies the power of the signal. The transmission signal then passes through the directional coupler 134, which distributes and supplies the transmission signal to the antenna 135 and the orthogonal demodulator 137. The signal is then radiated to space through the antenna 135.
The feedback signal having returned from the directional coupler 134 to the orthogonal demodulator 137 is frequency-converted into a baseband signal and further demodulated into an I/Q signal by the orthogonal demodulator. The feedback signal is further converted, by the A/D converter 138, into an analog feedback signal, which is then input to the subtractor 123, making up the distortion compensation circuit 120.
Here, in the distortion-compensated and amplified transmission apparatus 100A, the power amplifier 133 causes nonlinear distortion. However, the distortion compensation circuit 120 pre-distorts a signal corresponding to the nonlinear distortion caused by the power amplifier 133 to cancel and compensate for the distortion caused by the power amplifier 133. In the distortion compensation circuit 120, the OFDM signal generated by the OFDM modulation section 110 is input to the complex multiplier 125 and to the address generation section 121 and the delay circuit 122. A distortion compensation coefficient corresponding to the power of an input OFDM signal is accumulated in the LUT 124 in order to compensate for the distortion characteristic of the power amplifier 133. In the distortion compensation circuit 120, the address generation section 121 obtains an address corresponding to the power value of the OFDM signal to output the distortion compensation coefficient corresponding to the address obtained, to the complex multiplier 125. That is, the OFDM signal input to the complex multiplier 125 is complexly multiplied by the distortion compensation coefficient from the LUT 124. The resultant signal is then output. The signal is then subjected, by the D/A converter 131, to a process of converting the digital signal into an analog signal and, by the orthogonal modulator 132, to a process of orthogonally modulating the signal and then converting the orthogonally modulated signal into a radio frequency signal. The signal is then amplified by the power amplifier 133, and the resulting high-frequency transmission signal is transmitted through the antenna 135.
The nonlinear distortion characteristic of the power amplifier 133 is compensated for by the distortion compensation coefficient from the LUT 124 by which the OFDM signal is complexly multiplied. However, the nonlinear distortion characteristic of the power amplifier 133 is known to be varied depending on the environment in which the apparatus is installed, such as temperature. Thus, for distortion compensation for a change in environment, the signal output by the power amplifier 133 is fed back to update the distortion compensation coefficient stored in the LUT 124. Specifically, a part of the signal amplified by the power amplifier 133 is input to the orthogonal demodulator 137 via the directional coupler 134. The orthogonal demodulator 137 converts this radio frequency signal into a signal of the original baseband frequency band and further demodulates the signal. The A/D converter 138 then converts the signal into a digital signal. The subtractor 123 then subtracts the digital signal from the corresponding signal from the delay circuit 122. An output of the subtractor 123 allows detection only of the nonlinear distortion component of an actual output signal from the power amplifier 133. That is, the LUT 124 may update the distortion compensation coefficient using the distortion component from the subtractor 123. The LUT 124 updates the distortion compensation coefficient so as to suppress the detected distortion component.
As a technique related to the above-mentioned transmission apparatus, an OFDM signal transmission apparatus is suggested which avoids the adverse effect of the nonlinear distortion caused by the transmission power amplifier, by performing mapping control such that power for each OFDM symbol is prevented from exceeding a reference value and controlling the amplitude of a carrier modulation signal or the amplitude subjected to IFFT to allow the power amplifier to operate within the range of a linear region.
And as another technique, a nonlinear phase distortion compensation apparatus is suggested which stores compensation information corresponding to pre-divided regions of a dynamic range of output power, in a plurality of storage regions depending on the output power from the power amplifier, so that phase control is performed by selecting one of the plurality of storage regions in association with the output power.